Technological Field
The present invention relates to an image forming apparatus.
Description of the Related Art
In an electrophotographic image forming apparatus in the related art, the following technology has been employed in order to charge a photoreceptor. That is, a voltage is applied to a charging member which is to be brought into contact with the photoreceptor so that a surface of the photoreceptor is charged by proximate discharge. In this technology, the charging member is pressed against the photoreceptor so that the charging member and the photoreceptor come into contact with each other. Application of a bias to the charging member causes the proximate discharge near a contact portion between the photoreceptor and the charging member, and application of an electric charge to the surface of the photoreceptor causes charging of the surface of the photoreceptor.
According to a technology to apply a direct-current bias (DC bias) to the charging member, it is required to set an applied voltage to a high value under conditions disadvantageous for charging such as a case where printing speed is high or a case where a film thickness of the photoreceptor is thick. However, when a gap between a photoreceptor potential before charging and the applied voltage is large, unevenness in charging occurs due to overdischarge, which results in image defects.
In a technology to apply an alternating-current bias (a bias in which an AC bias is superimposed on a DC bias) to the charging member, charging and elimination are repeated so that unevenness in charging does not occur even with overdischarge because of the elimination. However, in the technology to apply the AC bias, an amount of current flowing through the photoreceptor is larger than that in the technology to apply the DC bias, which leads to a problem that wastage due to deterioration of the photoreceptor increases. When the film thickness of the photoreceptor becomes thin due to the wastage, a function to hold the potential deteriorates and noise is generated in an image. Therefore, the photoreceptor needs to be replaced.
In order to solve such problems, the following technique has been proposed (for example, see JP 2003-270910 A). That is, a DC bias is applied to a charging member when an amount used of a photoreceptor is small so as to reduce wastage of the photoreceptor, whereas an AC bias is applied to the charging member when the amount used of the photoreceptor increases and the photoreceptor deteriorates so as to suppress image defects due to cutting unevenness and the like of the photoreceptor.
In order to further prolong the life of a photoreceptor, a film thickness of the photoreceptor may be thickened so as to increase a cutting allowance of a film thickness of the photoreceptor. In such a case, if a DC bias is applied when an amount used of the photoreceptor is small as in the related art, it is required to set a voltage high as described above. Accordingly, overdischarge occurs, which leads to unevenness in charging and image defects.